An established cell line derived from the ascites of a patient with serous cystadenocarcinoma of the ovary has been characterized. Features studied included morphology, ultrastructure, clonogenicity in soft agar, population doubling time, karyotype, and chemosensitivity. The results indicated that the cells growing in culture were malignant ovarian tumor cells. These cells retained the ability to form free-floating cysts in culture, which were also present in the original ascitic fluid. The cell line had a chromosome number of 80–92 with no distinct mode and 15 stable markers. At passage 4 the cell line showed resistance to doxorubicin [adriamycin (ADM)], phosphoramide mustard (PM), and cisplatin [cis-dichlorodiammineplatinum(II)] (CIS) but rapidly reverted to CIS sensitivity. At passage 25 the cell line was still resistant to ADM and PM, but by passage 59 sensitivity to these drugs appeared to have increased. Frozen cells from passage 15 onward are available.